There are numerous studies concerning the effects of low frequency noise on humans. While the results are somewhat subjective with respect to the effects of such noise, there is a growing tendency to reduce noise effects from industrial sources wherever possible to maintain goodwill with the end users and the public. The term low frequency as used herein refers to noise in the audible frequency range from about 20 Hz to several hundred Hertz. For reference purposes, the hearing range of most humans is typically about 20 Hz-20 KHz and termed the audible frequency range.
For reference purposes, the article prepared by the Renewable Energy Research Laboratory, Dept. of Mechanical and Industrial Engineering, University of Amherst (June 2002) entitled “Wind Turbine Acoustic Noise” describes the general noise and sound fundamentals. The complexity arises when various electrical, electromechanical and mechanical systems are operating in an integrated fashion, such as a wind turbine, where it is difficult to accurately pinpoint the exact source of any audible noise.
The sources of such industrial noise are numerous, however it is generally recognized that one such source of low frequency noise comes from the drive trains from electrical and electromechanical systems. As used herein, gearboxes or drive trains (also called transmissions) shall refer to systems that employ some form of assemblies that are used to transfer and/or convert torque and speed in conjunction with one or more shafts. Drive trains are commonly used in implementations in diverse fields that include aviation, maritime, transportation, industrial, and energy. For example, gearboxes are used for the rotors on helicopters, propeller shafts on ships, wind turbine shafts, vehicle transmissions, and various types of coupling involved with engines, motors and generators. The electrical drive trains typically transfer torque and examples include a motor that transfers torque to a load and a generator that uses a rotating shaft to generate electrical power.
Various forms of passive and active noise cancellation schemes have been used to lower or eliminate the low frequency noise with some limited success and often by complex designs. There is a perceived need for addressing the noise problems that has not been satisfactorily addressed.